A surface acoustic wave (SAW) device having a piezoelectric substrate on which a comb electrode (IDT) for exciting a surface acoustic wave is formed is used as a component for frequency adjustment and selection in mobile phones and the like.
The surface acoustic wave device is required to have a small size, a low insertion loss, and unnecessary wave impermeability, and a piezoelectric material such as lithium tantalate (LiTaO3; LT) or lithium niobate (LiNbO3; LN) is used.
Meanwhile, communication standards for mobile phones of or after the fourth generation have narrower frequency band intervals in transmission and reception and wider bandwidth. Under such communication standards, the piezoelectric material used for the surface acoustic wave device is required to have a sufficiently small characteristic variation due to temperature. Further, shoulder characteristics of filters and duplexers need to be extremely steep so that extra noise does not occur between bands, and a high Q value is required.
With respect to the material used for such a surface acoustic wave device, composite substrates containing a piezoelectric material and another material have been investigated. For example, Patent Document 1 discloses a composite substrate having a high speed sound film in which the sound speed of a propagating bulk wave is higher than the sound speed of an acoustic wave propagating in a piezoelectric film, a low sound speed film in which the sound speed of a propagating bulk wave is lower than the sound speed of an acoustic wave propagating in the piezoelectric film, and the piezoelectric film stacked in order on a support substrate, and an acoustic wave device using the composite substrate can increase the Q value.
Furthermore, Patent Document 1 describes that the piezoelectric film is formed by ion implantation as a method of manufacturing this composite substrate.
Further, Non Patent Document 1 discloses a method of bonding a lithium tantalate substrate implanted with H+ ions to a support substrate via a SiN film or an SiO2 film, and then separating the lithium tantalate substrate into two.